Viral Plaque Assay

Introduction to Plaque Assay

The viral plaque assay is a quantitative method used to measure the number of virus particles capable of infecting cells. It is commonly used in microbiology to study bacteriophages and animal viruses.

• Bacteriophage: A virus that infects bacteria.

• Plaque: A clear zone on a bacterial lawn where cells have been lysed by viral infection.

• T4 virus: A well-studied bacteriophage that infects Escherichia coli.

Key Points

• Purpose of Dilutions: Serial dilutions are performed to obtain countable numbers of plaques for accurate quantification.

• Calculating Plaque Numbers: The number of plaques is used to determine the concentration of virus in the original sample.

Formula

• Plaque Forming Units (PFU) Calculation:

Example

• If 50 plaques are counted on a plate from a 10-6 dilution with 0.1 mL plated:

Fungi

Macroscopic and Microscopic Features of Mold

Fungi are eukaryotic organisms that include molds and yeasts. Identification often relies on both macroscopic and microscopic characteristics.

• Macroscopic Appearance: Includes colony surface, color, and texture.

• Microscopic Appearance: Involves spore arrangement and hyphal structure.

• Surface and Reverse Pigment: Surface pigment is the color seen from above; reverse pigment is the color seen from the underside of the colony.

Key Points

• Common mold identification patterns include spore type and arrangement.

• Microscopic examination is essential for accurate identification.

Example

• Aspergillus species show distinctive conidial heads under the microscope.

Streptococcus Identification

Differentiation of Streptococcus Species

Streptococcus species are Gram-positive cocci that are classified based on hemolysis patterns and biochemical tests.

• Hemolysis Types:

  • Alpha hemolysis: Partial hemolysis, greenish discoloration.

  • Beta hemolysis: Complete hemolysis, clear zone around colonies.

  • Gamma hemolysis: No hemolysis.

• Biochemical Tests: Bacitracin sensitivity, CAMP test, and optochin sensitivity are used for further differentiation.

Key Points

• Bacitracin Sensitivity: Group A Streptococcus (Streptococcus pyogenes) is sensitive.

• CAMP Test: Used to identify Group B Streptococcus (Streptococcus agalactiae).

• Optochin Sensitivity: Used to identify Streptococcus pneumoniae.

Example

• Group A Streptococcus: Beta hemolytic, bacitracin sensitive.

• Group B Streptococcus: Beta hemolytic, CAMP test positive.

Spelling and Biochemical Association

• Correct spelling and association of hemolysis type and group are required for identification.

• Alpha Streptococcus: Not Group A or B; includes Streptococcus pneumoniae (Group C).

• Beta Streptococcus: Can be Group A (S. pyogenes) or Group B (S. agalactiae).

Enterococcus Identification

Differentiation from Streptococcus

Enterococcus species are Gram-positive cocci that are biochemically distinct from Streptococcus.

• Bile Esculin Test: Enterococcus can hydrolyze esculin in the presence of bile.

• Salt Tolerance: Enterococcus can grow in 6.5% NaCl.

Key Points

• Correct spelling and association with biochemical tests are required.

• Enterococcus faecalis: Bile esculin positive, salt tolerant.

Gram-Negative Bacilli and Enterobacteriaceae Identification

Differentiation of Gram-Negative Bacilli

Gram-negative bacilli are differentiated using biochemical tests and selective media. The Enterobacteriaceae family includes many medically important genera.

• CNA and MacConkey Agar: Used to select and differentiate Gram-negative bacteria.

• Biochemical Tests: Lactose fermentation, indole production, citrate utilization, and others.

Key Points

• Lactose Fermentation: Pink colonies on MacConkey agar indicate lactose fermenters.

• Indole Test: Detects ability to produce indole from tryptophan.

• Citrate Test: Determines ability to use citrate as a carbon source.

Example

• Escherichia coli: Lactose fermenter, indole positive.

• Klebsiella pneumoniae: Lactose fermenter, indole negative.

Mixed Unknown Identification

Identification in Mixed Cultures

Mixed unknowns require the use of selective and differential media, as well as biochemical tests, to identify multiple organisms present in a sample.

• CNA Agar: Selects for Gram-positive bacteria.

• MacConkey Agar: Selects for Gram-negative bacteria and differentiates lactose fermenters.

• Biochemical Key: A flowchart or dichotomous key is used to systematically identify unknowns.

Key Points

• Interpretation of colony morphology and biochemical test results is essential.

• Use of a dichotomous key helps narrow down possible organisms.

Example

• Gram-positive cocci on CNA agar and Gram-negative rods on MacConkey agar indicate a mixed culture.

Summary Table: Hemolysis and Streptococcus Grouping

Additional info: Academic context and examples have been added to clarify identification methods and biochemical tests relevant to the study guide topics.